ABSTRACT Obesity is reaching epidemic proportions, affecting 36% of the adult population in the United States. The first line of obesity treatment is caloric restriction (CR), although recidivism is common. Therefore, alternative options to CR need to be considered. As humans are constantly eating, daily prolonged fasting through time restricted eating (TRE) shifts the paradigm in obesity treatment. In humans, TRE imposes a daily fasting period (14-16 hours) by only allowing food intake during fixed daily eating window (~8 to 10 hours). By implementing TRE in a pilot study of healthy, overweight humans for 12 weeks, we found that TRE reduced weight and fat mass, with greater restriction of the eating window resulting in greater loss of fat mass and visceral fat. As actigraphy-measured physical activity and sleep measures did not change, we propose that prolonged fasting is the primary mediator of TRE- associated changes. We hypothesize that TRE with ad libitum intake presents a highly viable alternative to CR in reducing weight, fat mass and improving insulin sensitivity, as daily prolonged fasting reduces caloric intake and enhances metabolic flexibility?the adaptability of fuel selection given ambient fuel exposure. We will recruit obese, insulin-resistant humans with an eating window ?14 hours, who will be randomized to 1 of 3 groups: 1) TRE (8 hour eating window) with ad libitum intake, 2) CR with 15% reduction of caloric intake, 3) non-TRE with ad libitum intake for 12 weeks. Our specific aims include the following: Aim#1: Evaluate the effect of TRE with ad libitum intake on weight and body composition. Aim#2: Assess the effect of TRE with ad libitum intake on caloric balance. Aim#3: Assess the effect of TRE with ad libitum intake on metabolic flexibility. This project?s objective is to examine the role of prolonged fasting in TRE-associated metabolic changes. The scientific premise is that prolonged fasting associated with TRE reduces caloric intake and enhances metabolic flexibility?the adaptability of fuel selection given ambient fuel exposure. Therefore, TRE will enhance fat oxidation in the fasting state and carbohydrate oxidation in the fed state, to be more metabolically advantageous than CR without prolonged fasting. The significance is establishing TRE as a practical means of implementing prolonged fasting on a large scale and a viable alternative to CR without prolonged fasting, thereby transforming the treatment of obesity